AFAIK this started happening because gstreamer is enabled by default in v26 and newer, turning it off in about:config reduces CPU usage of 360p YT videos to sane levels (20-30% of a Phenom II core), though it also means mp4 video no longer works. Obviously that isn't the "correct" solution; I have a working VDPAU driver, is there any way to get Firefox to make use of that, or behave itself in general instead of burning my desktop up?